Challenge Experimental Forest
The Challenge Experimental Forest occupies 1,446 ha at the western edge of the Feather River District of the Plumas National Forest. Site quality is rated high to very high. The predominant overstory is 100-140-year-old, even-aged Pacific ponderosa pine, but mixed conifer and mixed hardwoods types are also present. The Experimental Forest was formally designated in 1942 but not activated until 1958 when it was enlarged to its present boundaries. It was chartered to study and demonstrate silvicultural approaches to managing young-growth forests at lower elevations on the west slope of the Sierra Nevada. Activities peaked during the 1960’s when a resident staff of 5 oversaw a series of regeneration cutting experiments. Subsequent research included the relationship of opening size to regeneration success and exploratory work in hardwood management. Currently active experiments center on understory fuel reduction, tree spacing and understory competition, and issues of sustainable productivity. Today the Forest is administered entirely out of Redding
Challenge Experimental Forest is located on the Plumas National Forest in Yuba County, California (lat. 39o 28’ N., long. 121o 13’ W.) on a drainage divide of the Yuba River surrounding the small community of Challenge. Access is by the Marysville-LaPorte and Oroville-Forbestown Roads. Elevations vary between 730 m. to 1,130 m. Slopes are less than 30 percent on over 80 percent of the area and aspects are predominantly west and south.
The climate is Mediterranean in that summers are warm and dry, and winters are cool and wet. Mean annual precipitation averages 1727 mm., 98 percent of which falls between October and May. Occasional snowfalls melt rapidly leaving the ground free of snow most of the winter. Annual temperatures normally range from the 6o C. in January to 21o C. in July. The combination of high precipitation and mild temperature produce exceptionally favorable growing conditions.
Most soils are old, red, and deep. Clayey, oxidic, mesic Xeric Haplohumults of the Challenge and Sites Soil Series cover most of the Experimental Forest. Parent materials are derived mainly from greenstone (metamorphosed basalt). Soils have been mapped both by the Cooperative Soil Vegetation Survey and the University of California at Davis, in cooperation with Yuba County. Soils of specific study sites have been described more intensively.
Pacific ponderosa pine (Society of American Foresters, Forest Cover Type 245) is the major forest cover type. Sierra Nevada Mixed Conifer (Type 243), California Black Oak (Type 246), and Pacific Ponderosa Pine-Douglas-fir (Type 244) types are also present. Common understory plants are deerbrush ceanothus, manzanita, tanoak, gooseberry, and bracken.
Long-Term Data Bases
Precipitation and maximum and minimum temperatures have been recorded at the Challenge Work Center, Plumas National Forest, since 1938. The timber was inventoried in1938 and 1939, and again in 1979.
Research – Past and Current
Early research sought answers to two major questions: (1) How to grow and harvest young-growth (80-100 years old) ponderosa pine to ensure adequate regeneration, and (2) How to dispose of logging slash to reduce fire hazard and ensure adequate regeneration. Both even-aged (clearcutting, seedtree, and shelterwood) and uneven-aged (group and single-tree selection) management systems were studied. Management of native California hardwoods, field teating of hybrid and introduced pine species, and amount and pattern of soil moisture depletion by individual trees were other important early studies. In 1998 the first California Long-Term Soil Productivity experiment was installed as part of the North American LTSP Research Network.
More recent investigations include:
Results are directly applicable to more than 800,000 ha. of low elevation, highly productive sites on the west slope of the northern Sierra Nevada.
- Determining the extent to which natural reproduction follows different methods of cutting, logging, slash disposal, and site preparation.
- Evaluating the success of various direct seeding practices including site preparation and rodent control.
- Determining the magnitude, periodicity, and germination characteristics of conifer and hardwood seed crops.
- Determining the growth of the residual stand and changes in stand structure under management
- Determining the effect of timber harvest on magnitude and duration of leaching losses of soil nutrients.
- Determining the effect of soil compaction and loss of organic matter on long-term soil productivity.
- Evaluating the effect of fertilization on soil chemistry and growth and foliar chemistry of mixed-conifer species.
- Determining effects of competition and fertilization on growth, biomass, and chemistry of ponderosa pine.
Major Research Accomplishments and Impacts on Management
Long-term results from a study of group selection demonstrated and quantified the effect on regeneration of openings of different sizes on growth rates and proportion of the mixed conifer species.
Regeneration of five species of conifers, three of hardwoods, and two of shrubs was evaluated for five different cutting methods in terms of seedling stocking, density, and height growth. For ponderosa pine, seedtree and shelterwood methods produced the highest stocking and density. Selection cutting methods were best for survival and establishment of sugar pine, white fir, and Douglas-fir. For all species seedling height growth increased from single-tree selection to clearcutting. However, shrubs were particularly dense after clearcutting and broadcast burning, and could be a major factor in establishing adequate regeneration of rapid growth potential in this method.
A study of initial spacing and shrub competition on growth and development of planted ponderosa pine showed that the influence of shrub competition in restricting tree growth is short lived if trees are planted on a highly productive site with intensive site preparation. After about 15 years growth of trees with a shrub understory begin to exceed that of trees free of shrubs. Stem volume production is expected to be about the same with and without a shrub understory in about 60 years.
Soil solution chemistry measured in the 17-year period following harvesting and reforestation indicates that nitrate continues to leach below the rooting zone until a continuous canopy of perennial vegetation develops (about 14 years). However, losses are essentially equal to nitrogen inputs from precipitation.
Soil compaction led to sizable losses of tree growth through the first 5 years of stand development (but the opposite is true on sandy soils). Loss of surface organic matter had little influence of tree growth, but led to appreciable erosion of the unprotected mineral soil (the effect was much greater than that of soil compaction). Isotopic analyses of 13carbon in branch samples indicate periods of physiological drought stress caused by treatment or by climate.
- Plumas National Forest
- University of California at Davis and Berkeley
The Experimental Forest offers abundant opportunities for research into shrub/tree interactions, stand dynamics of ponderosa pine from young plantations to 130 year old natural stands and long-term soil changes in soil productivity as a consequence of logging operations.
The Challenge Experimental Forest has no facilities. General merchandise can be obtained in the town of Challenge. The nearest accommodations are in Oroville (42 km. northwest) or Marysville (56 km. southwest).
Contact Address, Website Address, Location
Robert F. Powers, Manager
Pacific Southwest Research Station
3644 Avtech Parkway
Redding, CA 96002-9241